Photolithography is a technique for transferring images onto semiconductor or other substrates. There are two fundamental types of photolithography systems. A first type, referred to as image-projection lithography, uses master patterns, referred to as masks or reticles, and a projection system for projecting the image on the mask on a substrate. A second type of system, referred to as a maskless or direct-write system, forms images directly onto the substrate by scanning (or “writing”) beams of light on the substrate. Some maskless lithography systems and method use multiple beams for increasing throughput.
U.S. Pat. No. 6,133,986 of Johnson, U.S. Pat. No. 5,936,713 of Paufler et al., U.S. Pat. No. 5,691,541 of Ceglio et el., U.S. Pat. No. 5,900,637 of Smith, U.S. Pat. No. 6,312,134 of Jain et al., U.S. Pat. No. 6,493,867 of Mei et al., U.S patent application 2002/0153362 of Sandstorm et al., U.S. Pat. No. 6,399,261 of Sandstrom, and PCT application WO 00/42618 of Johnson are believed to provide an adequate description of the state of the art. These patents and patent applications are incorporated herein by reference.
Maskless lithography may use electrons, ions or electromagnetic radiation for writing on the substrate. In either case, modulators for modulating the writing signal intensities are needed. In case of electromagnetic radiation in the UV or visible wavelength region, each maskless lithography system includes a light modulator. Systems that utilize multiple light beams include a modulator that is capable of modulating many light beams simultaneously. There are two types of light modulator, the first one is reflective and the second is diffractive. Both include modulation elements, such as movable micro-mirrors that may be moved/manipulated such as to direct an incident light beam a certain direction. A common prior art modulator can turn each of its modulating elements “on” (the light beam is directed towards the substrate) or “off” (the light beam is reflected away from the substrate) and is referred to as a binary modulator. Such a modulator is manufactured by Texas Instruments and is known as DMD.
In many cases it is desired to perform multi-level modulation, as opposed to binary modulation. Multi level modulation allows producing pixels that have a large range of intermediate intensity values resulting from a selective combination of multiple intensity levels. The intermediate intensity values are usually referred to as gray level values. These gray level values can increase the resolution of the lithography system. In multi-level modulation each pixel may have R intensity values (where R is usually 4,8,16,32 or any power of two).
A first prior art method provides multi-level modulation by controlling the duration of the “on” state of each modulating element. Said modulation is also termed Pulse Width Modulation (PWM). Accordingly, higher pixel values result in longer “on” durations, and vice versa.
This prior art method greatly limits the throughput of the lithography system. As each modulating element has a certain response period T, the time that is required to modulate a single K-level pixel is K*T. For example, in a typical DMD the response period of a mirror is about 30 microseconds. Assuming that 256 gray levels are required then the pixel time is about eight microseconds, thus only about 130 frames can be recorded per second.
Another prior art method for providing multi-level modulation is described at U.S. Pat. No. 6,399,261 of Sandstrom, assigned to Micronic Laser Systems AB from Taby, Sweden. This patent describes a maskless lithography system that includes a light modulator that is able to perform analog signal based multi-level modulation. Each modulation element is driven by an analog signal and may provide multiple intensity levels. The modulator needs to be calibrated using an empirical calibration procedure whereas a series of test patterns are images and analyzed. The modulator is susceptible to manufacturing inaccuracies, modulator temperature. Micronic has recently presented a prototype of a 1 Mega-pixel analog spatial light modulator.
Electromagnetic radiation beams, such as light beams, may be characterized by their polarization. The electric field of a linearly polarized optical wave lies only in a single plane. The electric filed of a circularly polarized optical wave lies in two orthogonal planes and are phased shifted by a quarter wavelength (or an odd amount of quarter wavelengths) of the optical wave. Polarizing beam splitters divide an optical wave that has electric fields in two orthogonal planes into two orthogonally polarized optical waves. Phase retardation involves making an optical path length for one out of two orthogonal linear polarizations different than the other. Quarter wave retarders convert linearly polarized optical waves into circularly polarized optical waves and vice versa. Variable retarders are able to change their retardance and accordingly are able to change the relative phase shift between the electrical fields in two orthogonal planes, thus introducing a phase shift. Variable wave retarders may change their retardance between zero and a portion of a wavelength. Variable wave retarders are characterized by the maximal amount of phase shift they introduce. For example a half wavelength variable retarder is able to change its retardance between zero and half wavelength. Wave retarders such as but not limited to quarter wavelength retarder and polarizing beam splitters are known in the art.
There is a need to provide a high throughput system for facilitating high-speed multilevel modulation for improved mask making and wafer lithography.